WASHINGTON — If Barack Obama goes on to win the Democratic nomination for president, historians may point to the Feb. 12 "Potomac primaries" as the tipping point that established him for the first time as the clear front-runner.

Not only did the Illinois senator win the three primaries, making him undefeated in the last eight contests, he won them by massive margins. His 64-to-35 percent victory in Virginia was particularly stunning to the campaign of Hillary Rodham Clinton, which thought she might come close or even possibly win in Old Dominion.

But Senator Obama defeated Senator Clinton in just about every demographic category there, demonstrating that he is expanding into her base support of low-income voters and women. Obama also appears to be winning over the supporters of former Sen. John Edwards (D) of North Carolina, who departed the race two weeks ago. In the early contests, he had done well among white men, union households, and the economically stressed, all groups where Obama is building support.

On Wednesday morning, Obama was to take his message on the economy – the No. 1 issue for voters – to the General Motors assembly plant in Janesville, Wis. According to his campaign, the senator planned to discuss his "agenda to restore economic balance and fairness" and "create millions of new jobs." The economy will be a dominant theme of his stump speeches in coming weeks, campaign aides say.

Obama was already in Wisconsin Tuesday night to deliver his victory speech before a crowd of 16,000 people at the University of Wisconsin, Madison. Wisconsin's primary is next Tuesday, Feb. 19, along with caucuses in Hawaii. Obama is expected to win both, which puts tremendous pressure on Clinton in the next round, the March 4 primaries in Texas, Ohio, Vermont, and Rhode Island. The first two are especially critical, analysts say, along with the Pennsylvania primary on April 22.

Many analysts, including longtime Clinton friend and adviser James Carville, have opined that if she loses any of the big three – Ohio, Texas, or Pennsylvania – her campaign is effectively over.

Obama himself seemed to pivot to a general election stance on Tuesday night, taking aim at the Republicans' likely nominee and trying to lash him to the unpopular Republican president.

"John McCain is an American hero; we honor his service to our nation," Obama said. "But his priorities don't address the real problems of the American people, because they are bound to the failed policies of the past. George Bush won't be on the ballot this November, but his war and his tax cuts for the wealthy will."

Obama also took a moment to savor his Tuesday victories. In addition to Virginia, he won Maryland 59 to 37 percent and the District of Columbia 75 to 24 percent. Obama now holds the overall lead in delegates, though not by much – 1,223 to 1,198, according to the Associated Press. A candidate needs 2,025 delegates to win the Democratic nomination.

Clinton is still very much in the hunt – and no one counts out the Clintons until the bitter end – but Obama has big momentum from Tuesday's primaries.

Exactly how she can counter Obama, though, is less than clear. Her campaign appears in disarray; the deputy campaign manager's resignation was announced as the election returns were coming in Tuesday night. Clinton's husband, the former president, cannot reprise the role he played last time she was in trouble, dominating her campaign's message with racially tinged rhetoric, analysts say.

"She's in a very difficult spot," says Del Ali, head of the nonpartisan polling firm Research 2000. "She's got to go after him, but she can't use the blueprint from South Carolina."

Senator McCain, who won all three of the Republican primaries Tuesday, also appeared to target Obama in his victory speech. He riffed on one of Obama's signature themes, hope, turning it to his own message of service to America.

"My hope for our country resides in my faith in the American character...," he said. Then, he took aim at Obama: "To encourage a country with only rhetoric rather than sound and proven ideas that trust in the strength and courage of free people is not a promise of hope. It is a platitude."

McCain's victories on Tuesday, beating out top rival Mike Huckabee, erased some of the embarrassment of having been beaten in two out of three GOP contests last weekend. But the presumed GOP nominee's relatively low margin of victory in Virginia – 50 to 41 percent – served as a reminder that he still has work to do in uniting the Republican coalition behind his candidacy.

In Virginia, McCain lost to the former Arkansas governor among self-identified evangelicals and conservatives. Some 46 percent of the Virginia GOP electorate identified as evangelical, and of those, Huckabee beat McCain by a whopping 60 to 31 percent. Conservatives made up 65 percent of GOP voters in Virginia, and they supported Huckabee 51 to 38 percent.

Another sign of distress in Virginia for Republicans was the continuing pattern during the 2008 primary season of Democratic turnout dwarfing the GOP's by 2 to 1. That show of Democratic enthusiasm does not bode well for the Republicans in the fall.